This invention relates to digital frequency adjustment means suitable for use in an electronic timepiece, particularly a digital electronic timepiece, for the purpose of adjusting to a desired value the output frequency of a frequency divider circuit in an electronic timepiece of the type which displays time by using the divider circuit to divide the frequency of a standard oscillator circuit, the output of the divider circuit then being used to drive time display means.
In recent years the utilization of ICs in digital electronic timepiece circuitry has become quite pronounced, and in particular there has been rapid development toward the adoption of large scale integration (LSI) in which single-function IC groups are mutually wired on a circuit board and closely integrated. However, where electronic circuits are constructed through IC and LSI techniques, an extremely important problem in view of the accurate production and operation of semiconductor devices is reducing the number of terminals of the semiconductor devices which construct the electronic circuitry. Moreover, in order to allow this electronic circuitry to drive an electro-optical display device such as liquid crystal cells or light emitting diodes, which possess a comparatively large number of terminals, a very large proportion of the terminals which the electronic circuitry possesses must be expended to provide a connection to the electro-optical display device.
It is desirable to reduce the number of circuit terminals in order to conserve space since most timepiece components such as the electro-optical display device, the electronic circuitry, crystal oscillator and battery must be accommodated in the limited confines of a watch case, especially if the case is designed for a wristwatch. However, reducing the number of circuit terminals in present electronic wristwatches can entail reducing the number of electrodes that provide a connection to the display device and the control switches, a sacrifice that has been difficult to make since this necessarily limits timepiece operability and function. In addition, there is a tendency today toward the production of digital electronic timepieces which possess a number of functions such as chronograph, worldtime and calculator functions, a condition which calls for an ever greater number of control switch terminals while the number of circuit terminals has also steadily increased for the same reasons. Hence, it has been extremely difficult to reduce the cost and size of the electronic circuitry and assure the reliability of its connections.
Under these circumstances, frequency adjustment in a conventional electronic timepiece, in which oscillation is produced by a crystal oscillator, was accomplished by varying the capacity of a variable condenser connected to the oscillator circuit which made it possible to adjust its frequency. In the limited confines of an electronic wristwatch, however, a sufficient adjustable range could not be obtained since it was necessary to make use of a variable condenser which was small in size. Another defect resides in the fact that the temperature-humidity characteristics of the oscillator circuit vary widely depending upon environmental conditions and the passage of time due to the instability of the variable element.
In an effort to overcome these defects there have been proposed a number of so-called digital frequency adjustment systems in which the output frequency of a frequency divider circuit is adjusted by varying its adjustment ratio while leaving the frequency obtained from the oscillator circuit untouched. However, all of these systems are disadvantageous in that they require providing a large number of terminals for setting the adjustment ratio in order to allow the output frequency of the divider circuit to be adjustable over a wide range.